There are many types of communications networks, for conveying information between remote points. For example wide area networks belonging to telecommunications providers, internet service providers, company intranets, cable television distribution systems and other data communication networks can use optical networks in which digital information is conveyed in the form of optical signals through optical fibers. Digital information in communication networks can be categorised as asynchronous or synchronous types. Synchronous types such as SDH (Synchronous Digital Hierarchy) require a common timing reference to operate accurately. That is, the clocks in one node of the network should operate at the same speed as the clocks in other nodes of the network.
To provide for a common timing reference, communication networks carrying digital information can include synchronization networks, whose job it is to ensure that a common timing reference is used throughout the communications network. One such synchronization network is described in European Telecommunication Standards Institute (ETSI) document European Guide (EG) 201 793 v1.1.1 (2000-10), entitled “Transmission and Multiplexing (TM); Synchronization Network Engineering”. This document describes the various elements that make up a synchronization network, and the principles of operation by which such a network distributes accurate timing information from so-called Primary Reference Clocks (PRCs) to the clocks located in other pieces of equipment throughout the network. PRCs are the highest quality clocks in a network, and are usually based on a free-running Caesium Beam oscillator giving a very accurate clock performance.
Poor network synchronization usually leads to large amounts of jitter and wander, and, consequently, to transmission errors and buffer under/overflow. Both these faults will result in service problems causing high error rates and service unavailability. In the best case, then, poor synchronization causes only few inconveniences to any other network layer; in the worst case, it can make the entire telecommunication network stop passing traffic. A well-planned and maintained synchronization network is then a prerequisite for avoiding or reducing a risk of critical failures in traffic networks.
The planning of a synchronization network is typically performed manually, according to some rules as defined in relevant ITU-T recommendations (e.g., ITU-T G.803) and other relevant standards (e.g. the above mentioned ETSI EG 201 793). Some computer-aided tools may assist the synchronization network planning and maintenance by, for instance, supporting the off-line design of the reference timing signals distribution, as well as providing simulations of the synchronization network normal operation and fault scenarios.
The management of a live synchronization network, on the other hand, is usually distributed over a number of platforms, as the types of the equipment participating into the synchronization network can be different. Therefore separate network management systems have to coexist, each taking care of a network type, e.g., synchronization dedicated, switching, transmission, and so forth. These management systems usually only provide the means to monitor the synchronization network and detect possible failures; in the latter case the single nodes will rearrange according to their synchronization set-up, or it will be up to the operator to perform recovery actions.
In case of a physical layer based synchronization network, the synchronization network is typically non-dedicated, meaning it is superimposed on the communications network and the transport layer of this network is the carrier of the reference timing signal. Due to that, although this is the most commonly used method, some issues may arise: this type of network is generally complicated to plan, difficult to operate, might be dependent on other operators and on the type of communications network it is superimposed on.
Accurate planning of non-dedicated synchronization networks is generally a complicated task. Even if this is done perfectly in the initial planning, it requires significant effort in the re-planning of the synchronization network every time other network types/layers are changed.
Additional issues may arise when deploying new and heterogeneous technologies. For instance, synchronization networks can be hybrid networks made up of trails having some parts using synchronous communications and other parts using packet-based communications. For example, nodes supporting either TDM legacy (e.g., PDH, SDH), or synchronous Ethernet, can be mixed with those using packet based technologies to transfer the synchronization information. The synchronization network management, in this case, appears extremely challenging and may significantly increase the OPEX (operational expenditure) for an operator.
It is known from the Precision Time protocol (PTP) (IEEE standard 1588-2008 Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems) to send timing information over packet networks, so that carrier Ethernet networks, for example, can carry telecom quality clocks to network nodes. This standard defines a structured time stamp based on the output of a master clock at a time of transmission of the timing packet, and an Ethernet or IP packet location for it. PTP uses a two way transfer technique to enable frequency, time and phase alignment to be generated by slave nodes. It defines master nodes, boundary nodes and transparent nodes. The packets can be passed by any Ethernet nodes and be treated as regular data packets.